This invention relates to an optical system for use of laser processing machine.
Recently, laser scribing has been attracting the interest of researchers in the field of electronic device manufacture. There are two kinds of processes which can be employed to produce patterns on thin films by irradiating with a laser beam. According to one process, a laser beam is shaped by a mask in advance of being focussed to the film through an optical system, so that the film is processed in accordance with the image determined by the mask. By virtue of the optical system, the focussed beam comes to have a necessary energy density to process the film. The optical system is normally composed of spherical lenses. According to the other process, an optical system is utilized to squeeze an orignial laser beam into a narrow beam having a prescribed spot cross section with its sufficient energy density. By repeating the irradiation of a spot laser beam while the irradiation position on the film is shifted with respect to the laser beam, a groove can be engraved. Spherical lenses are also suitable for this case. However, objective lenses for microscope can be used, the wavelength of the laser beam permitting.
Usually, such an optical system is comprised of a plurality of spherical lenses having different focal lengths in order to obtain a small aberration without compromising the numerical aperture of the lenses.
On the other hand, a laser beam can be shaped into a flat thin beam having a linear cross section by making use of a cylindrical lense while the cross section area of the beam is expanded if necessary. In this case, a single lense is employed which comprises an incidence side convex surface and an opposing outlet side convex surface whose curvature is smaller than that of the incidence surface. The spherical aberration is limited in light of such a configuration.
However, when a large numerical aperture is desired, the spherical aberration has to be compromised.